In the field of magnetic resonance (MR) imaging, it is known to employ MR radio frequency (RF) antennae that are provided for transmitting RF power at a resonance frequency for resonant excitation of nuclei in a subject of interest during a first period of operation, and for receiving MR RF energy emitted by the nuclei at a second period of operation that is different from the first period of operation.
In the state of the art of operating an MR imaging system, periods of transmitting RF power and periods of receiving magnetic resonance imaging signals are taking place in a consecutive manner during an MR imaging session. The periods of transmitting RF power and periods of receiving MR imaging signals may be generated by controlling the MR antenna by transmit/receive (T/R) switches.
T/R switches of the prior art may apply PIN (positive intrinsic negative) diodes as switching elements. Typically, a low RF impedance, which is formed by a forward-biased PIN diode, is transformed via a quarter-wave transmission line into a high RF impedance as its dual at a corresponding connection point. T/R switches of this type are, for instance, described in FIG. 2.11 of Microsemi-Watertown: “The Pin Diode Circuit Designers' Handbook”, 1998, DOC. #98=WPD-RDJ007, Microsemi Corporation, 580 Pleasnt Street, Watertown, Mass. 02472, USA. In order to achieve sufficient RF isolation between an RF amplifier providing RF power for resonant excitation and a preamplifier at a receiving end during a period of receiving MR imaging signals, the quarter-wave transformation has to be repeated in several stages. In most cases, two stages provide sufficient RF isolation.
Since the transmission lines required for the quarter-wave transformation tend to be very long and bulky, they can be replaced by corresponding networks exhibiting the same transformation properties. As a consequence, each component of this network is given.
A typical embodiment of a prior art T/R switch is shown in FIG. 1a. FIG. 1b illustrates an equivalent lumped-element circuit employing a π-network, wherein required inductor L and capacitors C are individually exactly determined. In FIG. 1a, an RF input line TX′ is providing RF power for the resonant excitation of nuclei from an RF amplifier (not shown). The RF input line TX′ is connected to an MR RF antenna M′ via a PIN diode D, which in turn is connected to a quarter-wave RF transmission line (λ/4). A distal end of the quarter-wave RF transmission line is connected both to a receiving port RX′ with an RF pre-amplifier provided to amplify received MR signals, and to another PIN diode D′. The PIN diodes can be transferred between a state of low RF impedance and a state of high RF impedance by controlling a DC bias current, whose providing circuitry is not shown for clarity reasons.
As can be gathered form FIG. 1b, the RF isolation per stage of the receiving port RX′ during periods of transmitting RF power is mainly given by a voltage divider formed by an inductance L and a low RF impedance of diode D′. Since both values are fixed in principle by the π-network equivalent to the quarter-wave transmission line (λ/4), the RF isolation per stage is also determined.
Typically, one stage allows for an RF isolation during periods of transmitting RF power of about 40 dB. If an RF power level of e.g. 62 dBm (approx. 1.6 kW) is applied to the receiving (RX′) port, there is not enough safety margin to reliably protect the RF pre-amplifier (only 62 dBm−40 dB=22 dBm at a maximum allowable power level of about 25 dBm). For this reason, a second stage ST′ has to be added, which increases complexity and physical size of the T/R switch. Additionally, a higher total bias current has to be provided.
For reasons shown, present T/R switches typically consist of two stages in order to achieve the required isolation. An example of a prior art two-stage T/R switch and its equivalent lumped-element circuit is shown in FIGS. 1c and 1d, respectively. They comprise a large number of elements or bulky transmission lines requiring a high total DC bias current.
With an increasing interest in local multi-element MR RF antenna arrays provided as well for transmitting RF power for resonant excitation as for receiving MR imaging signals, there is a growing need for improved transmit/receive switches. For these, it is desirable to provide improved RF isolation both during the periods of transmitting RF power and during periods of receiving MR imaging signals. Other objectives are to provide more design freedom for T/R switches and to lower the total required DC bias current in relation to the RF isolation effect.